How can I set processor affinity to a thread or a Task in .NET?

Question

Can we set two threads or two tasks to execute with different processor affinity in a C# application?

I have read about SetThreadAffinityMask, but have found no example of how that should be used.

Alternatively, is there any way for TPL (Task Parallel Library) to execute two threads/Tasks with high priority to use 100% CPU?

Answer 1

Process and ProcessThread objects have a ProcessorAffinity property of IntPtr type that can be directly manipulated to read/change affinity for up to 64 processors:

using System.Diagnostics;

Process proc = Process.GetCurrentProcess();
long affinityMask = 0x000F; // use only any of the first 4 available processors
proc.ProcessorAffinity = (IntPtr)affinityMask;
 
ProcessThread thread = proc.Threads[0];
affinityMask = 0x0002; // use only the second processor, despite availability
thread.ProcessorAffinity = (IntPtr)affinityMask;

You can also use the thread's IdealProcessor property to allow the scheduler to prefer running the thread on a specified processor (without guarantee).

Yes, it's that easy :)

Reference: ProcessThread.ProcessorAffinity Property

Answer 2

Actually, .NET Framework and Windows manage the threads pretty well, distributing them evenly on every processor. However, the distribution of threads can be manipulated manually using Process and ProcessThread.

using System;
using System.Diagnostics;
using System.Threading;

namespace ThreadTest
{
    class Program
    {
        static void Main(string[] args)
        {
            //Get the our application's process.
            Process process = Process.GetCurrentProcess();

            //Get the processor count of our machine.
            int cpuCount = Environment.ProcessorCount;
            Console.WriteLine("CPU Count : {0}", cpuCount);

            //Since the application starts with a few threads, we have to
            //record the offset.
            int offset = process.Threads.Count;
            Thread[] threads = new Thread[cpuCount];
            Console.WriteLine(process.Threads.Count);
            LogThreadIds(process);

            //Create and start a number of threads that equals to
            //our processor count.
            for (int i = 0; i < cpuCount; ++i)
            {
                Thread t = new Thread(new ThreadStart(Calculation))
                { IsBackground = true };
                t.Start();
            }

            //Refresh the process information in order to get the newest
            //thread list.
            process.Refresh();
            Console.WriteLine(process.Threads.Count);
            LogThreadIds(process);

            //Set the affinity of newly created threads.
            for (int i = 0; i < cpuCount; ++i)
            {
                //process.Threads[i + offset].ProcessorAffinity = (IntPtr)(1L << i);
                //The code above distributes threads evenly on all processors.
                //But now we are making a test, so let's bind all the threads to the
                //second processor.
                process.Threads[i + offset].ProcessorAffinity = (IntPtr)(1L << 1);
            }
            Console.ReadLine();
        }
        static void Calculation()
        {
            //some extreme loads.
            while (true)
            {
                Random rand = new Random();
                double a = rand.NextDouble();
                a = Math.Sin(Math.Sin(a));
            }
        }
        static void LogThreadIds(Process proc)
        {
            //This will log out all the thread id binded to the process.
            //It is used to test whether newly added threads are the latest elements
            //in the collection.
            Console.WriteLine("===Thread Ids===");
            for (int i = 0; i < proc.Threads.Count; ++i)
            {
                Console.WriteLine(proc.Threads[i].Id);
            }
            Console.WriteLine("===End of Thread Ids===");
        }
    }
}

Now check the task manager, we can see that that the second processor is taking all the work loads. The task manager window

Answer 3

Process.GetCurrentProcess().ProcessorAffinity = (System.IntPtr)2; 

Answer 4

Actually OS is capable of load balancing your cores/processors but if you want to do it explicitly use mentioned via PInvoke. You pass id of thread (not managed one!) and mask - the bit array of cores.

Answer 5

The following example from MSDN shows how to set the ProcessorAffinity property for an instance of Notepad to the first processor.

using System;
using System.Diagnostics;

namespace ProcessThreadIdealProcessor
{
    class Program
    {
     static void Main(string[] args)
        {
        // Make sure there is an instance of notepad running.
        Process[] notepads = Process.GetProcessesByName("notepad");
        if (notepads.Length == 0)
            Process.Start("notepad");
            ProcessThreadCollection threads;
            //Process[] notepads; 
            // Retrieve the Notepad processes.
            notepads = Process.GetProcessesByName("Notepad");
            // Get the ProcessThread collection for the first instance
            threads = notepads[0].Threads;
            // Set the properties on the first ProcessThread in the collection
            threads[0].IdealProcessor = 0;
            threads[0].ProcessorAffinity = (IntPtr)1;
        }
    }
}